A typical vehicle speed detection system for detecting the ground speed of a vehicle uses a Doppler sensor. This detection system is designed to emit electromagnetic waves (radio waves or ultrasonic waves) from the vehicle onto the ground at a specified beam depression angle .theta. and to receive the electromagnetic waves which have been reflected off the ground. Based on these radiant and received waves, Doppler shift (beat wave frequency) f.sub.d is calculated, using the following equation. EQU f.sub.d =2f.sub.t.multidot.vcos .theta./C (1)
In this equation, f.sub.t is transmission frequency, C is electromagnetic wave propagation velocity (light velocity or sonic velocity) and v is actual vehicle speed.
This prior art vehicle speed detection system, however, suffers from the problem that when the actual speed of a vehicle is sensed by the Doppler sensor mounted on the vehicle body, the movements (pitching, vertical movement, yawing) of the vehicle body affect the Doppler sensor causing errors in speed detection so that the vehicle speed detection system cannot detect accurate ground speed.
Attempts to solve this problem are proposed in Japanese Patent Laid-Open Publication No. 53-17376 (1978) and No. 61-201176 (1986) which are directed to eliminating the effect of pitching angular velocity which most significantly affects speed detection. According to Japanese Patent Laid-Open Publication No. 53-17376, a plurality of Doppler sensors are mounted on a mobile unit and the speed of the mobile unit is determined by averaging, through mutual compensation, a plurality of Doppler signals sensed by the plurality of Doppler sensors. The detection system disclosed in Japanese Patent Laid-Open Publication No. 61-201176 is designed to calculate an error in speed detection from the angular velocity of the Doppler sensor with respect to the center of gravity of the vehicle and to correct a sensed ground speed based on the calculated error value.
Generally, the installation level of such a Doppler sensor, that is, the level at which such a Doppler sensor is installed when mounted on a vehicle body, is determined according to the specification of the sensor that is determined by the output power of the sensor.
Japanese Patent Laid-Open Publication No. 53-17376 directed to compensating for errors in speed detection due to the pitching of a vehicle requires a plurality of Doppler sensors, which leads to an increase in the manufacturing cost of the entire system. Japanese Patent Laid-Open Publication No. 61-201176 has the disadvantage that when the detection system is applied to a tracklaying vehicle such as bulldozers, the center of rotation of the vehicle varies according to the kind of operation being performed and to the terrain of the ground where the vehicle is traveling, which adversely affects the accuracy of correction.
Regarding the installation level of the Doppler sensor, if the Doppler sensor is mounted at a level in a vehicle such as a construction machine in accordance with the specification of the sensor, there is a strong likelihood that mud etc. may stick to the sensor causing a decrease in the signal strength of the sensor or the sensor may get damaged, hitting against rocks or stones.
It is conceivable that these problems may be solved by installation of a higher power sensor at a high position in a vehicle. However, use of a higher power sensor not only leads to higher cost but also causes disturbance to other working vehicles being operated in the same yard because of strong electromagnetic waves radiated from the high power sensor. Additionally, the output of the sensor higher than the specified value does not meet the requirement stipulated by the regulation.
The present invention is directed to overcoming the above problems and a first object of the invention is therefore to provide a vehicle speed detection system capable of correcting errors due to pitching of a vehicle body at low cost. A second object of the invention is to provide a vehicle speed detection system incorporating a Doppler sensor which can be installed at high levels in a vehicle without increasing the output power of the sensor.